In order to attain high strength, polyester fibers and polyester films are subjected to as high a drawing as possible. The drawing ability of the polyester is significantly improved through a narrow molecular weight distribution and a low concentration of low-molecular weight molecules, such as water, ethylene glycol and oligomers. The effect of the low-molecular compounds is based both on their reactivity, for example, hydrolysis through water, as well as on their function as plasticizers.
Polyesters, subsequently to their production by means of melt phase polycondensation, are either processed directly or first pelletized. Then, depending on the use, before processing, the pellets are crystallized and, in the solid phase at temperatures of above 100.degree. C., subjected to a post-condensation and/or dealdehydization and/or drying. The diffusion and the reaction ability of the low molecular compounds and split-off products is critical for this thermal post-treatment. As a result of the core-to-shell structure of the pellet grains, this effect differs from the center to the surface of the pellet to a pronounced degree. The post-treatment, consequently, leads to a broadening of the distribution of molecular weights, which is undesirable for fibers and films. This effect is particularly intense in the solid phase condensation of polyethylene terephthalate to a final intrinsic viscosity of approximately 0.90 to 1.05 dl/g for spinning into tire cord.